3. Trumps :set diffopt+='horizontal' right now; is this the right decision?

I would think that it should work the other way around. Right now, diffopt can be used if the user wants to treat diff splits as special, and do something different with them (e.g. always open them vertically). I think this philosophy should continue, and if the user always wants horizontal diffs but in general likes their splits to be vertical, they can set splitvertical and diffopt+=horizontal to achieve this.

On an unrelated note, you may want to move this discussion to your talk page, or maybe even a subpage of that. In general the user page itself provides a short, static bio, and the discussion takes place on the talk page. --Fritzophrenic 17:50, October 4, 2010 (UTC)